It is known in the art that amorphous polyesters can be extruded with crystallizable polyesters to improve performance of the film. Performance improvements over monolayer crystallizable polyester films include high temperature sealing, solvent sealing, and lower cutting force during trimming. Crystallizable polyesters used as a heat-sealing layer often results in poor performance. Amorphous polyester resins cause problems when being handled as waste material (hereinafter sometimes called "regrind") which is generated during processing. When mixed with crystalline polyester pellets and dried at a temperature above the glass transition temperature of the amorphous polyester, the regrind softens and sticks together, forming large clumps. These clumps make drying of the regrind very difficult and cause significant problems with air flow in a dryer and also when feeding to an extruder.
Some techniques for overcoming this sticking problem include melting the regrind with crystallizable polyesters using devices specially equipped for this process and then reforming the extrudate into pellets that can later be thermally crystallized. Although this technique may alleviate the sticking problems, the cost can be relatively expensive.
This invention provides a polyester composition which offers advantages due to its heat-sealability, while at the same time crystallize to sufficient levels to allow mixing and drying with crystallizable polyesters.
Polyesters containing repeat units from terephthalic acid, ethylene glycol and cyclohexanedimethanol are known in the art. For example, see U. S. Pat. Nos. 4,373,002; 4,091,150; 4,405,400; 4,011,358; 4,765,999; 4,399,179; 4,946,743; 4,375,494 and 2,901,466. Some of the films disclosed in these patents are oriented. In others, the heat-sealing layer is not as described and claimed herein. For example, U.S. Pat. No. 4,765,999 discloses a dual layered film wherein the heat-sealing layer is a polyester having repeat units from terephthalic acid, ethylene glycol and 1,4-cyclohexanedimethanol. This heat-sealing polyester contains a greater number of repeat units from 1,4-cyclohexanedimethanol than claimed herein, making the heat-sealing layer too difficult to crystallize and thus, would result in sticking when mixed with other polyesters during crystallization.
Japanese Patent 62,222,845 discloses a laminated heat-sealable polyester film comprised of (1) a crystallizable PET containing up to 10 mol % of comonomer that has a heat of fusion of at least 7 cal/g and (2) a PET copolymer containing 10-20 mol % isophthalic acid that has a heat of fusion of up to 5 cal/g. The film is oriented and used for pouches for boiling food for sterilization. An important relation between the layers is the refraction index. A preferred range is disclosed for optimum properties. This patent does not address regrind nor the crystallizability of the second layer. Furthermore, it does not mention the use of PET copolymers containing 1,4-cyclohexanedimethanol.
Japanese Patent 60,253,545 discloses a laminated polyester film for shrink-packaging comprised of (1) a copolyester modified with 5 to 50 mol % 1,4-cyclohexanedimethanol and (2) PET. The total thickness of the copolyester layer is 20-70% of the entire structure. An important property of the film is shrinkage. Again, this patent does not address regrind nor the crystallizability of the copolyester layer.